1. Field of the Invention
This invention relates to a push-button telephone dialing device, more particularly to a push-button telephone dialing device which incorporates a telephone hold function circuit.
2. Description of the Related Art
As the number of telephone sets in a single household increases, so does the need to provide each telephone set with an additional telephone hold function circuit. Presently, there are two methods which can be employed so as to incorporate the telephone hold function in a telephone set. In the first method, an additional telephone hold circuit is connected to a telephone dialing integrated circuit of the telephone set. The required number of components is increased, thereby increasing the complexity and cost of the system. In the second method, the telephone hold function circuit is incorporated in the telephone dialing integrated circuit. The size of the telephone dialing integrated circuit is increased because two additional pins (the hold function input pin and the hold function output pin) are required. Moreover, the hold function input signal employed in the second method is not derived from the push-button keypad of the telephone set and thus does not undergo a debouncing process to obviate any erroneous signal. Therefore, the untimely entry and release of the telephone set from the telephone hold state caused by erroneous signals cannot be prevented.
Most telephone sets are also provided with a "hands-free" dialing circuit to permit the execution of a dialing action without lifting the telephone handset. In a conventional telephone dialing integrated circuit, which incorporates both hands-free dialing and telephone hold circuits, the operation of one function is completely independent and does not affect the operation of the other function.
To reduce the electrical power consumption of conventional telephone sets, the status of the telephone handset (off hook or on hook) is used to determine when electric power should be supplied to the telephone dialing integrated circuit. Thus, dialing and communication can be achieved only when the telephone handset is an off hook condition. No keypad input signal can be received by the telephone dialing integrated circuit when the telephone handset is in an on hook condition. From the foregoing, it can be concluded that the hands-free dialing function is incompatible and cannot be incorporated in a keypad input scan system. The keypad input scan system can be used with the telephone hold function since the telephone hold function is generally deployed when the telephone handset is in an off hook condition.